# What is a memory?

An episode of [Science for Kids / Science's Not Boring: The Kids Science Podcast](https://invisiblenetwork.ai/podcast/1896928296), 5 Sep 2026, 15 min.

Audio: https://www.buzzsprout.com/2624612/episodes/19720826-what-is-a-memory.mp3

Have you ever wondered why you can forget where you put your shoes, but you never forget how to ride a bike? Your brain is the ultimate supercomputer! In fact, it can hold about 2.5 million gigabytes of information—that is like recording three million hours of your favorite TV shows! Join Mira and Finn on Science is Not Boring as they uncover the amazing secrets of how our memories actually work. Did you know that every time you learn something new, your brain literally grows new physical branches? We will explore how a clever scientist used a giant, squishy sea slug to prove it! You will also meet the most famous patient in neuroscience history, Patient H.M., who helped doctors discover that we actually have two completely different memory systems. Plus, find out the sneaky biological reason why a simple smell, like baking cookies or rain on the sidewalk, can instantly transport you back in time! Get ready to build some brand new brain connections and discover the incredible science happening right inside your very own head!

## Transcript

[0:00] SPEAKER_01: Welcome to Science Is Not Boring by Kidopoly.com.
[0:03] SPEAKER_01: I'm Mira! And I'm Finn! Hey Finn, think about your favourite video game console, or maybe a laptop computer.
[0:12] SPEAKER_01: How much storage does it have?
[0:14] SPEAKER_01: A few hundred gigabytes maybe?
[0:16] SPEAKER_01: Maybe a terabyte if it's really fancy?
[0:19] SPEAKER_00: Yeah, about that.
[0:21] SPEAKER_00: My tablet ran out of space just from downloading a few movies and games.
[0:25] SPEAKER_00: Why?
[0:26] SPEAKER_01: What if I told you that your brain, the squishy organ sitting right inside your skull, can hold about 2.5 million gigabytes of information?
[0:36] SPEAKER_00: 2.5 million?
[0:38] SPEAKER_00: No way! That's impossible!
[0:41] SPEAKER_01: It is absolutely true! Scientists call that measurement 2.5 petabytes.
[0:47] SPEAKER_01: To put that into perspective, if your brain was a digital video recorder, it could hold 3 million hours of television shows.
[0:55] SPEAKER_01: Wait, 3 million hours?
[0:57] SPEAKER_01: How long would it take to watch all that?
[0:59] SPEAKER_01: You would have to leave the television running continuously, 24 hours a day, for over 300 years to watch it all.
[1:07] SPEAKER_00: Haha, that is the biggest hard drive ever.
[1:11] SPEAKER_00: I don't think I'll live for 300 years.
[1:13] SPEAKER_01: Exactly! And today, we're exploring exactly how this ultimate supercomputer works.
[1:19] SPEAKER_01: How do our memories actually function?
[1:22] SPEAKER_01: And why do we forget some things but not others?
[1:25] SPEAKER_00: Oh, like why I can completely forget where I put my shoes yesterday, but I never forget how to ride a bike.
[1:32] SPEAKER_01: You hit the nail on the head! We're going to explore how memories are physically made, the incredible scientists who figured it out, and the clever anatomical tricks that help us remember more.
[1:43] SPEAKER_01: So, let's start with the basics.
[1:45] SPEAKER_01: A lot of people think memories are just floating around in your head, like pictures dumped into a bucket.
[1:52] SPEAKER_01: But they are actually physical structures built by your brain cells.
[1:57] SPEAKER_01: Brain cells?
[1:58] SPEAKER_01: You mean neurons, right?
[2:00] SPEAKER_01: You got it! The average human brain has about 86 billion neurons, and they talk to each other through tiny chemical connections called synapses.
[2:11] SPEAKER_00: 86 billion?
[2:13] SPEAKER_00: That's more than 10 times the number of people on Earth.
[2:17] SPEAKER_00: All inside my head.
[2:19] SPEAKER_01: Yes, and those 86 billion neurons form over 100 trillion synapses.
[2:26] SPEAKER_01: Every single time you learn something new, whether it's a math fact or the lyrics to a song, those connections actually change.
[2:35] SPEAKER_00: Wait, my brain literally changes its physical shape when I learn.
[2:40] SPEAKER_01: Yes, but proving that was really hard.
[2:43] SPEAKER_01: Because human brains are so dense and complicated, scientists needed something much simpler to study.
[2:50] SPEAKER_01: So what did they use?
[2:51] SPEAKER_00: A lab mouse?
[2:52] SPEAKER_00: Or maybe a fruit fly?
[2:54] SPEAKER_01: Nope.
[2:55] SPEAKER_01: In the 1960s, an amazing scientist named Dr.
[2:59] SPEAKER_01: Eric Candle decided the best way to study memory was to look at a sea slug.
[3:06] SPEAKER_00: A sea slug?
[3:07] SPEAKER_00: Why a slug?
[3:09] SPEAKER_00: They don't really seem like geniuses.
[3:11] SPEAKER_01: Well, he used a specific marine snail called Aplysia californica, and he chose them for two incredible reasons.
[3:20] SPEAKER_01: First, instead of 86 billion neurons, the slug only has 20,000 neurons in its entire nervous system.
[3:28] SPEAKER_00: Okay, 20,000 is a lot easier to map out than 86 billion.
[3:33] SPEAKER_00: What was the second reason?
[3:35] SPEAKER_01: The sheer size of them! Human neurons are microscopic.
[3:40] SPEAKER_01: But this sea slug has giant neurons.
[3:43] SPEAKER_01: Some are a full millimetre in diameter.
[3:46] SPEAKER_00: A millimetre?
[3:47] SPEAKER_00: That means you can see an individual brain cell with just your bare eyes.
[3:52] SPEAKER_01: Exactly! You didn't even need a super powerful microscope.
[3:56] SPEAKER_01: Dr.
[3:57] SPEAKER_01: Candel could look right at these giant cells and watch exactly what happened when the slug learned something new.
[4:04] SPEAKER_01: So, Dr.
[4:04] SPEAKER_01: Candel wanted to see if learning physically changes the nervous system.
[4:09] SPEAKER_01: He would give the sea slug a mild, harmless shock, then tap its siphon, a little tube on its back.
[4:16] SPEAKER_01: Did it hurt the slug?
[4:18] SPEAKER_01: What did it do?
[4:19] SPEAKER_01: It didn't hurt at all.
[4:21] SPEAKER_01: But the shock startled the slug.
[4:23] SPEAKER_01: It would instantly pull its delicate gills inside, kind of like a scared turtle pulling its head into its shell to hide.
[4:31] SPEAKER_01: That makes sense.
[4:32] SPEAKER_01: It's just trying to protect itself.
[4:34] SPEAKER_01: Right! From that shock, the slug learned that its environment was a threat.
[4:39] SPEAKER_01: It became super sensitive, instantly hiding its gills every single time it was gently tapped.
[4:46] SPEAKER_00: Ah, so it remembered the experience.
[4:48] SPEAKER_01: It formed a memory.
[4:50] SPEAKER_01: Exactly.
[4:51] SPEAKER_01: But here is the wild part.
[4:53] SPEAKER_01: Dr.
[4:54] SPEAKER_01: Candal actually looked closely at the slug's giant neurons before and after it learned this new memory.
[5:01] SPEAKER_01: What did he see?
[5:02] SPEAKER_00: Did the neurons light up or change colour?
[5:04] SPEAKER_01: They physically grew! When the slug formed a new memory, its neurons literally sprouted new physical branches at the synapses.
[5:14] SPEAKER_00: Whoa! Like a tree growing new roots in the soil.
[5:17] SPEAKER_01: Yes, they reached out across the gap and made brand new, permanent connections with other neurons.
[5:24] SPEAKER_01: A memory isn't just invisible computer software, it's a real, physical bridge built out of proteins inside your brain.
[5:33] SPEAKER_00: That is mind-blowing.
[5:35] SPEAKER_00: So when I sit in school and memorise a new spelling word, my brain is actually sprouting tiny little branches.
[5:42] SPEAKER_01: You're literally building brand new brain tissue.
[5:45] SPEAKER_01: Dr.
[5:46] SPEAKER_01: Candel proved that memory involves anatomical changes.
[5:50] SPEAKER_01: His work was so groundbreaking that he won the Nobel Prize in Medicine for it in the year 2000.
[5:57] SPEAKER_00: All thanks to a giant squishy sea slug.
[6:00] SPEAKER_00: That is so cool.
[6:02] SPEAKER_00: I'll never look at a slug the same way again.
[6:04] SPEAKER_01: But discovering how a memory is built led scientists to another huge question.
[6:10] SPEAKER_01: We know how the physical connections form, but where do they go?
[6:14] SPEAKER_01: Does the brain have a specific storage room for memories?
[6:17] SPEAKER_01: To figure out where memories are stored, we have to talk about the most famous patient in the entire history of neuroscience.
[6:25] SPEAKER_01: His real name was Henry Mollison, but for decades, scientists just called him patient HM to protect his privacy.
[6:34] SPEAKER_01: What happened to him?
[6:36] SPEAKER_00: Did he have a brain injury?
[6:38] SPEAKER_01: Back in the year 1953, Henry was suffering from terrible, life-threatening seizures.
[6:44] SPEAKER_01: So, a surgeon named William Scoville performed an experimental surgery and removed a part of Henry's brain called the hippocampus.
[6:54] SPEAKER_01: The hippocampus?
[6:55] SPEAKER_01: What does that part even do?
[6:58] SPEAKER_01: Well, they didn't really know back then.
[7:01] SPEAKER_01: The surgery successfully cured his seizures, but it had a shocking and tragic side effect.
[7:07] SPEAKER_01: Henry completely lost the ability to make new memories.
[7:12] SPEAKER_00: Wait, he couldn't remember anything at all.
[7:15] SPEAKER_01: He remembered his childhood perfectly, and he remembered facts from before 1953.
[7:22] SPEAKER_01: But if he met you, had a nice conversation, and you left the room for just five minutes, when you came back, he'd have absolutely no idea who you were.
[7:32] SPEAKER_00: Oh wow.
[7:33] SPEAKER_00: That's like being permanently stuck in time.
[7:36] SPEAKER_01: It really was.
[7:38] SPEAKER_01: But then, a brilliant neuropsychologist named Dr.
[7:41] SPEAKER_01: Brenda Milner started working with him.
[7:44] SPEAKER_01: And through Henry, she discovered something incredible about how our brains categorize information.
[7:51] SPEAKER_00: What did she do to figure it out?
[7:53] SPEAKER_01: She gave Henry a special test.
[7:56] SPEAKER_01: She asked him to trace a five-pointed star on a piece of paper.
[8:00] SPEAKER_01: But he couldn't look directly at his hand.
[8:03] SPEAKER_01: He could only see his hand's reflection in a mirror.
[8:06] SPEAKER_01: It is super confusing for your brain to process.
[8:11] SPEAKER_00: Oh, I've tried that.
[8:13] SPEAKER_00: It's so hard to draw backward.
[8:15] SPEAKER_00: You always move your pencil the wrong way.
[8:17] SPEAKER_01: Exactly.
[8:19] SPEAKER_01: Henry did this test every single day for three days.
[8:23] SPEAKER_01: And because he had no hippocampus, every time Dr.
[8:26] SPEAKER_01: Milner gave him the test, he swore he had never seen it before in his life.
[8:32] SPEAKER_00: Because he couldn't form new memories.
[8:34] SPEAKER_01: Right.
[8:35] SPEAKER_01: But here is the crazy part.
[8:38] SPEAKER_01: Even though he didn't remember practicing, his hand got better at it every single day.
[8:44] SPEAKER_01: By day three, he drew the star perfectly, with almost zero mistakes.
[8:49] SPEAKER_00: What?
[8:50] SPEAKER_00: How is that even possible if his brain couldn't remember doing it?
[8:54] SPEAKER_01: Dr.
[8:55] SPEAKER_01: Milner realised we have two totally different memory systems.
[8:59] SPEAKER_01: The hippocampus makes declarative memories.
[9:02] SPEAKER_01: Those are facts, names, and events, like what you ate for breakfast.
[9:07] SPEAKER_00: And what's the other system?
[9:09] SPEAKER_01: It's called procedural memory.
[9:11] SPEAKER_01: That's the memory of how to do physical things, like riding a bike, tying your shoes, or drawing a star in a mirror.
[9:19] SPEAKER_01: That lives in a totally different part of the brain, like the basal ganglia and the cerebellum.
[9:25] SPEAKER_01: Because Henry's cerebellum was untouched, his procedural memory worked perfectly, even though his fact memory was broken.
[9:33] SPEAKER_01: That's exactly why you can forget someone's name, but you never forget how to ride a bike.
[9:38] SPEAKER_00: That makes so much sense.
[9:40] SPEAKER_00: It's like my brain has two different filing cabinets.
[9:43] SPEAKER_00: But why do I forget where I put my shoes yesterday?
[9:47] SPEAKER_01: Because your brain is actually designed to forget.
[9:50] SPEAKER_01: If you remembered every single detail of every single day, every leaf on every tree you walked past, your brain would be completely overloaded.
[10:00] SPEAKER_00: So forgetting things is actually a good thing.
[10:02] SPEAKER_00: It's on purpose.
[10:04] SPEAKER_01: Yes.
[10:05] SPEAKER_01: During the night, while you sleep, your brain actively prunes away the weak connections.
[10:10] SPEAKER_01: It clears out useless information so you have physical room for the memories that actually matter.
[10:16] SPEAKER_00: Like emptying the trash bin on a computer to free up hard drive space.
[10:20] SPEAKER_01: Exactly.
[10:22] SPEAKER_01: Now, are you ready for a fun fact about memory that will make you go, whoa?
[10:26] SPEAKER_01: Always! Hit me with it! Have you ever smelled something, like a specific cookie baking or the smell of rain on a hot sidewalk, and you were instantly transported back to a memory from when you were little?
[10:38] SPEAKER_00: Yes.
[10:39] SPEAKER_00: The smell of those waxy crayons always reminds me of my first day of kindergarten.
[10:44] SPEAKER_00: Why does smell do that better than seeing something?
[10:47] SPEAKER_01: It's all because of your brain's anatomy.
[10:50] SPEAKER_01: When you see or hear something, the signal goes to a relay switchboard in your brain called the thalamus before it gets sorted to other areas.
[10:59] SPEAKER_01: Okay, but what about when you smell something?
[11:02] SPEAKER_01: Smell skips the switchboard entirely.
[11:05] SPEAKER_01: Your olfactory bulb, which processes smells, has a direct, straight-shot superhighway to the amygdala, which handles emotion.
[11:13] SPEAKER_00: And the hippocampus, which handles memory.
[11:16] SPEAKER_01: Bingo! Smell is the only sense with a direct, unblocked physical link to the memory and emotion centres of your brain.
[11:23] SPEAKER_01: It takes just one or two synapses for a scent to trigger a deeply emotional memory.
[11:29] SPEAKER_00: That's why the memory hits you so fast and feels so real.
[11:33] SPEAKER_00: That is unbelievably cool.
[11:35] SPEAKER_00: Alright, Finn, it is time for the quiz.
[11:37] SPEAKER_00: Are you ready?
[11:38] SPEAKER_00: Okay, I'm ready.
[11:40] SPEAKER_00: Let's see if my hippocampus was paying close enough attention today.
[11:44] SPEAKER_00: First question.
[11:45] SPEAKER_01: How many petabytes of data can the human brain hold?
[11:52] SPEAKER_01: Next one, think back to our marine friend.
[11:55] SPEAKER_01: What animal did Dr.
[11:56] SPEAKER_01: Eric Candel study to learn about memory?
[12:03] SPEAKER_01: Question 3.
[12:04] SPEAKER_01: What specific part of the brain did patient HM lose, which stopped him from forming new fact memories?
[12:14] SPEAKER_01: Here's number four.
[12:16] SPEAKER_01: What type of memory helps you ride a bicycle without having to think about it?
[12:24] SPEAKER_01: Last question.
[12:26] SPEAKER_01: Which of your five senses has a direct superhighway straight to the memory centre of your brain?
[12:36] SPEAKER_01: Great job! Let's see how you did.
[12:39] SPEAKER_01: The first answer is the brain can hold 2.5 petabytes, which is equivalent to 2.5 million gigabytes.
[12:48] SPEAKER_01: For question 2, Dr.
[12:49] SPEAKER_01: Candel studied the giant neurons of a sea slug, specifically Aplesia californica.
[12:56] SPEAKER_01: Number 3.
[12:57] SPEAKER_01: The answer is patient HM lost his hippocampus.
[13:02] SPEAKER_01: Question 4.
[13:03] SPEAKER_01: Remembering how to perform a physical task, like riding a bike, is called procedural memory.
[13:10] SPEAKER_01: And the final answer is your sense of smell has a direct, unblocked link to your memory centres.
[13:17] SPEAKER_01: How did you do on the quiz, Finn?
[13:19] SPEAKER_01: I bet your procedural memory for taking tests is getting really sharp.
[13:24] unknown: Ha!
[13:24] SPEAKER_00: I definitely built some new synapses today.
[13:27] SPEAKER_00: It's amazing to think that our brains are just constantly growing, building branches and pruning themselves every single day.
[13:35] SPEAKER_01: It really is.
[13:36] SPEAKER_01: It makes you realise that every new thing you learn, every book you read, and every new place you visit literally changes the physical structure of who you are.
[13:46] SPEAKER_00: And hey, if you listeners learned something new today and built some new brain branches with us, you should definitely leave us a five-star review.
[13:55] SPEAKER_01: Yes, please do.
[13:57] SPEAKER_01: Tell us in your review if you would rather have the vast memory capacity of an elephant or the giant 1mm neurons of a sea slug.
[14:05] SPEAKER_01: Just scroll down on your podcast app and tap those five stars.
[14:09] SPEAKER_00: And don't forget to check out our site, kidopoly.com.
[14:13] SPEAKER_00: We've got tons of fun learning games, cool activities, and more awesome science stuff there.
[14:19] SPEAKER_01: Absolutely! Head over to our site kidopoly.com to keep exploring and growing those neurons.
[14:26] SPEAKER_01: Oh, and if you want your name shouted out on the show, or just want to say hi, send us an email at hello at kidopoly.com.
[14:34] SPEAKER_00: We love reading your emails and hearing what topics you want us to explore next.
[14:39] SPEAKER_01: Thank you all so much for exploring the supercomputer inside your skull with us today.
[14:44] SPEAKER_01: See you next time on Science Is Not Boring.
[14:48] SPEAKER_01: Bye everyone.
[14:49] SPEAKER_01: Keep learning.

Shows are listed from Apple Podcasts' charts and directory, and Podcast Index where it is used. Every show and episode belongs to its publisher and plays from the publisher's own feed; this site hosts none of them.